This invention relates to the field of prosthetics, and more particularly, to an intervertebral disc prosthesis designed to replace a damaged intervertebral disc.
The human spine consists of twenty-four small bones known as vertebrae, or “vertebral bodies,” that protect the spinal cord and provide stability to the torso. The vertebrae are arranged in a column and stacked vertically upon each other. Between each vertebra is a fibrous bundle of tissue called an intervertebral disc. These intervertebral discs act as a cushion to the spinal column by absorbing energy and transmitting loads associated with everyday movement. They also prevent the vertebrae from rubbing against each other.
Each intervertebral disc comprises two distinct regions. A firm outer region, the annulus, maintains the shape of the intervertebral disc. An inner region, the nucleus, provides a resilient tissue that enables the disc to function as a shock absorber. Over time, the normal aging process causes the intervertebral discs to degenerate, diminishing their water content and thereby reducing their ability to properly absorb the impact associated with spinal movements. Diminished water content in the intervertebral discs may also cause the vertebrae to move closer together. Tears and scar tissue can weaken the discs, resulting in injury. When the discs wear out or are otherwise injured, a condition known as degenerative disc disease results. With this condition, discs do not function normally and may cause pain and limit activity.
The condition of degenerative disc disease can potentially be relieved by a surgical procedure called artificial disc replacement. In this procedure, the damaged intervertebral disc is replaced by a prosthetic disc. One well known intervertebral prosthetic disc is produced by DePuy Spine, Inc. of Raynaham, Mass. and is sold under the trademark CHARITÉ®. This disc prosthesis is comprised of two metal endplates and a center polyethylene core. The center core includes a superior spherical bearing surface and an inferior spherical bearing surface. The superior endplate includes a concave surface that fits upon and is congruent with the superior bearing surface of the core. The inferior endplate includes a concave surface that fits under and is congruent with the inferior bearing surface of the core. During the CHARITÉ® artificial disc replacement procedure, the damaged disc is first removed via an anterior surgical approach and the end surfaces of the exposed vertebrae are cleared of debris. The vertebrae are spread apart and the metal endplates are positioned on the respective vertebra and tapped into place. The polyethylene core is then inserted between the endplates and the vertebrae are returned to their normal position. The pressure of the spinal column further seats the endplates into the vertebral bones and secures the core in place.
Many patients receiving artificial discs would benefit from restricted movements between the vertebral bodies that sandwich the artificial disc. For example, a surgeon may determine that a particular patient would be benefit from limited lateral bending movement between two vertebral bodies. One reason for limiting lateral bending or other movements may be to protect the facet joints. Thus, it would be advantageous to provide an intervertebral disc prosthesis configured to limit movement in certain planes.
In addition to limiting certain movements, it would be advantageous if the artificial disc capable of providing limited movement could be provided by simply modifying one component of the artificial disc. For example, for a three-piece disc arrangement having a superior endplate, an inferior endplate and a core, it would be advantageous if one of several different cores could be selected to accomplish the desired degree of limited movement or desired type of limited movement for the disc. Such an arrangement would allow standard endplates to be used along with different core configurations in order to provide a plurality of different prosthetic devices offering different movement characteristics.
Furthermore, it would be advantageous if the means for providing limited movement of the endplates relative to the core could be accomplished without the endplates contacting one another. Such an arrangement would prohibit potentially damaging contact between the endplates and provide a more natural endplate movement. Accordingly, it would be advantageous to provide an intervertebral disc prosthesis having a shear-limiting core.